In a typical internal combustion engine, a greater efficiency and power output can be achieved by raising the compression ratio of the cylinders to maintain the peak combustion pressure and peak unburned gas temperature. A higher compression ratio can also improve exhaust emissions indirectly. On the other hand, a higher cylinder pressure can create problems with engine knock. The typical engine configuration is, therefore, a compromise between these two limits. In an effort to work around this compromise and achieve significantly higher compression ratios while still avoiding knock, the concept of a variable compression piston has been developed. This design is used to maintain the prescribed peak combustion pressure in a cylinder regardless of varying engine loads.
The typical configurations of prior art variable compression pistons are shown in SAE technical paper "Variable Compression Pistons," Cedric Ashley, SAE technical paper number 901539, 1990. The typical variable compression piston has an outer piston slidably and concentrically mounted on an inner piston which is rotatably mounted on a connecting rod. The reciprocating motion of the inner piston is, therefore, fixed relative to the cylinder, but the outer piston motion can vary due to its ability to move up or down axially relative to the inner piston. The typical variable compression piston also has an upper cavity between the upper surface of the inner piston and the lower surface of the outer piston which is filled with oil from the crankcase; a lower cavity formed between the lower surface of the inner piston and some member fixed to the lower end of the outer piston which is also filled with oil; and a mechanism for transmitting oil into, out of or between the two cavities in response to the current compression ratio in the cylinder.
One such mechanism is a hydraulic system in which an oil supply line runs up through the connecting rod and into the piston pin where it forks into two one way valves, one valve connected to the upper cavity and the other connected to the lower cavity, to supply oil from the crankcase to the system. This hydraulic system also has an orifice located at a point along the bottom of the lower cavity to allow oil to drain back into the crankcase. The system further has two discharge valves connected to the upper cavity, one primary and the other secondary with the secondary discharge valve having a higher spring rate than the primary. The two discharge valves drain the oil back into the crankcase. In this system the primary discharge valve is calibrated to operate the system both at steady state and for increasing or decreasing loads on the engine, while the secondary discharge valve only opens when there is a large increase in the engine load, to more quickly drain oil from the upper cavity.
Several potential problems exist with such a system. First, the orifice in the lower cavity may allow air to enter the system which will have an adverse impact on the system operation since air is compressible. A second potential problem is that the oil supply line has no control over the excess or lack of oil pressure that may occur. If the oil pressure is zero or negative, then the oil may reverse direction and drain out of the oil supply line leaving an inadequate supply of oil for the cavities. This condition may occur when the piston is at bottom dead center. On the other hand, if the oil pressure is too high, the pressure from the supply oil entering the cavities may interfere with the calibration of the opening of the discharge valves leading to an incorrect piston height. This system has no control over the maximum oil pressure that is supplied to the cavities when the piston reaches top dead center.
Another such variable compression piston has a hydraulic system in which an oil supply line runs up through the connecting rod and into the piston pin through a one way valve connected to the upper cavity, to supply oil from the crankcase to the system. This system also has a bore, with an orifice for limiting the rate of flow, through the inner piston connecting the upper oil cavity to the lower oil cavity allowing free flow in both directions and a bore from the upper oil cavity to the lower cavity with a one way valve mounted in it that allows oil to flow from the upper chamber to the lower chamber when the oil pressure in the upper chamber is larger than the oil pressure in the lower chamber by a minimum threshold amount. These two bores allow oil to be supplied to the lower cavity. The system further has a single discharge valve connected to the upper cavity, which discharges oil into the crankcase, with a spring rate calibrated to change the oil volume in the upper chamber based upon the compression pressure. This system further has a one way valve mounted along the oil supply line within the connecting rod which closes when the pressure in the oil line drops below a certain minimum amount in order to prevent the oil from draining out of the supply line and leaving the system without any supply oil.
This system also has several potential problems which exist. First, the bore extending between the upper oil cavity and the lower cavity not only allows free flow of fluid between the two cavities thereby preventing potential cavitation problems in the lower oil cavity, but also allows the high pressures which build up in the upper oil cavity to be transmitted to the lower oil cavity. The net effect is much higher pressures in both cavities since the cross sectional area of the lower face, which sees the pressure, is subtracted from the cross sectional area of the upper face of the upper oil cavity to yield a lesser effective pressure area than the area of the upper face alone. A second problem with this system is its limited ability to discharge oil from the upper oil cavity at a rapid rate should the engine load dramatically increase with only a one discharge valve design. A third problem is similar problem that the first design also has. This system has no way of limiting the maximum oil pressure in the oil supply line that feeds into the upper oil cavity, so again if the pressure reaches too high of a level the calibration of the system can be compromised. A further drawback to both of these prior art configurations and others is that none of them teach a method or develop structure in which the calibration of the discharge valve can be made to cause the compression ratio to increase with increasing engine speed or decrease with decreasing engine speed.